The copy of the letter Fairfax Media obtained was addressed to Greens Senator Nick McKim and cc’d to federal Education Minister Simon Birmingham.

“One example of the level of risk is to consider that, strictly speaking, the Australian Government is only required to honour the last official agreement it has with Tasmania,” Mr Rockliff writes.

“This was the original level of funding allocated under Gonski in 2013 including indexation, and the Australian Government may choose to index at zero - this would be a diabolical loss for Tasmanian schools.”

Mr Rockliff writes the legislation is “truly needs based and sector blind” and he is “pleased that the Gonski 2.0 proposal includes continuous uplift in federal funding overtime”.

“Labor and the Union claims that Tasmanian schools stand to lose money under Gonski 2.0 are false because they are not based on actual budgeted figures,” he writes.

“The reality is, Tasmania stands to lose substantially, if no new deal is agreed and this would be a genuine and dramatic loss."

Ms O’Byrne said the Liberals were “holding schools to ransom”.

“Jeremy Rockliff’s letter to the Greens is a disgrace,” she said.

“Mr Rockliff should be lobbying his federal colleagues for the amount of funding Tasmania was originally promised, not negotiating on their behalf for an inferior deal.

“The Minister admits he is ‘disappointed’ the deal on the table is not as good as the one both sides of politics signed up to originally.”

In response to the letter, Senator McKim said the Greens “want the best possible outcomes for Tasmanian students, in particular those at schools with the highest need”.

Mr Rockliff did not respond to questions on Monday. The Australian Education Union Tasmania was contacted for comment.